This invention relates to a communication system such as an ATM-LAN, a server and an address management method. More particularly, the invention relates to a communication system equipped with a plurality of terminals, a server having an ATM address table for storing the corresponding relationship between the protocol address and ATM address of each terminal, and an ATM exchange which accommodates each terminal and the server.
Improvements in the performance of personal computers and work stations have been accompanied by the rapid proliferation of applications for dealing with high-speed data and multimedia. In addition, such techniques as remote file access and decentralized computing have been established though use of LANS. For these reasons, there is expected to be greater demand for higher speed LANs for multimedia purposes.
In order to satisfy these expectations, there has been increased activity in the research and development of LANs (ATM-LANs) using ATM communication. An ATM-LAN is a switching LAN in which terminals are connected in a star-like configuration to an apparatus (an ATM exchange, for example) having an ATM switching function. Each terminal establishes a virtual channel (VC) directed toward a destination terminal and performs a data transmission by means of a fixed-length packet, referred to as an ATM cell, comprising a five-byte header and 48-byte data. As a result, it is possible to set a number of VCs on a terminal interface (where the number is capable of being expressed by the VPI/VCI) so that the terminal is capable of communicating with a plurality of other terminals simultaneously via the set VCs.
When communication is performed on a LAN, it is generally required that the originating terminal know the physical address of the terminal of the other party. In a conventional LAN, the physical address is a MAC address. Data in a LAN is transmitted in frame units, with each frame containing the addresses of the originating and terminating terminals. FIG. 21 is a diagram of a frame in a case where the protocol of a LAN is TCP/IP. The frame includes a start delimiter (SD) and an end delimiter (ED) between which are placed a destination address DA serving as a control field and a layer-2 address (MAC address), a sending address SA, an information field I (IP packet) and a frame scanning sequence FSC. The IP packet is subdivided into a control information field, a destination address DA′ serving as a protocol address (IP address), a sending address SA′ and an information field I′.
Communication with another terminal is not possible if the MAC address of the terminal is not known. If the MAC address of a party's terminal is unknown (but the protocol address is known), therefore, the original terminal determines the MAC address of the higher-order protocol address (IP address) by an address resolution protocol (ARP) and performs communication upon disassembling the higher-order packet (IP packet) into frames using the MAC address.
FIG. 22 is a processing flowchart of the ARP. If the MAC address of another party's terminal is unknown (NO at step S1), the originating terminal creates a frame (ARP frame) and sends the frame to the LAN (step S2). The ARP frame contains {circle around (1)} a broadcast address as the destination MAC address DA of the frame, {circle around (2)} a protocol address of another party's terminal serving as the destination protocol address DA′, and {circle around (3)} an indication that the frame is an ARP frame, this serving as control information CF′.
Upon receiving this ARP frame, each terminal determines whether the frame has its own address or is a broadcast frame. In this case, the frame is a broadcast frame and therefore the frame is accepted (step S11). Next, after verification of the fact that the frame is an ARP frame, each terminal determines whether the protocol address of the destination agrees with its own protocol address (step S12). Processing is terminated if the two addresses disagree. If the two addresses do agree, however, an answer frame which contains the terminals own MAC address is created at sent back to the originating terminal (step S13). Upon receiving the answer frame (YES at step S3), the originating terminal registers the correspondence between the MAC address of the other party's terminal contained in this answer frame and the protocol address in an internal IP-MAC address table (step S4) and then creates a frame (see FIG. 21) using the other party's MAC address and sends this frame to the LAN to perform LAN communication (step S5).
The foregoing is for the case of connection-less communication. In an ATM-LAN based upon connection-type communication, a VC is established between terminals and communication is performed while forming a higher-order packet into cells. In such an ATM-LAN, a physical address corresponds to the address (VPI/VCI) of the ATM layer, and this ATM layer address (VPI/VCI) is decided by a set-up sequence using the ATM address of the terminal. A set-up sequence is a call set-up processing sequence executed at the time of an outgoing call.
Thus, when the ATM address of the other party's terminal is unknown in an ATM-LAN, communication cannot be performed. This necessitates a method of analyzing the ATM address based upon the higher-order protocol address.
Conventional methods of acquiring the ATM address of another party's terminal include a broadcast method of broadcasting an interrogation request to all terminals and a server method of providing a server within the ATM-LAN and interrogating the server.
FIG. 23 is a diagram for describing the broadcast method. Terminals 1a, 1b, 1c are connected to an ATM-LAN 2. According to the broadcast method, the ATM address interrogation request is broadcast within the network if the ATM address of the other party's terminal is unknown at the moment a communication request is generated, and a prescribed terminal responds to this interrogation by answering with its own ATM address. For example, if the ATM address of terminal 1b is unknown to the terminal 1a when this terminal communicates with the terminal 1b in FIG. 23, the terminal 1a broadcasts an ATM address interrogation request cell (which has the protocol address of terminal 1b) within the network. The terminal 1b, which is that terminal having a protocol address identical with that contained in the received cell, answers the terminal 1a with its own ATM address. The terminal 1a uses the received ATM address to call the terminal 1b and communicate with it.
FIG. 24 is a diagram for describing the server method. FIG. 24 shows the terminals 1a, 1b, 1c and the ATM-LAN 2, which in this case is provided with a server 3. According to the server method, the corresponding relation between protocol addresses and ATM addresses of all terminals is registered in an ATM address table in advance. If the ATM address of another party's terminal is unknown at the moment a communication request is issued, an ATM address interrogation request is sent to the server and the server answers by retrieving the ATM address from the ATM address table. For example, if the ATM address of terminal 1c is unknown to the terminal 1a when this terminal communicates with the terminal 1c in FIG. 24, the terminal 1a sends the server 3 an ATM address interrogation cell (which has the protocol address of terminal 1c). The server 3 answers the terminal 1a by retrieving the ATM address of terminal 1c from the ATM address table, and the terminal 1a uses the received ATM address to call the terminal 1c and communicate with it.
If the ATM address of another party's terminal is unknown at the moment a communication request is issued in the broadcast method, a broadcast is made within the network and the terminal is interrogated directly. Unlike the server method, implementation is easy because it is unnecessary to create a data base in advance. However, in cases where frequent communication is made with an unspecified terminal whose ATM address is unknown, broadcast of the interrogation request is made whenever a communication request is issued. A problem which arises as a consequence is an increase in control traffic. This problem becomes particularly pronounced with an increase in the number of terminals or depending upon the scale of the network.
With the server method, on the other hand, there is no increase in traffic because there is no broadcast of an interrogation request. However, it is required that information (the correlation between protocol addresses and ATM addresses) regarding all terminals connected to the ATM-LAN be registered in the ATM address table in advance. At the time of initial installation therefore, it is necessary that the information regarding all terminals be registered in a data base (ATM address table) and that the data base be updated whenever a terminal is added on or moved. Further, an increase in the number of terminals or an increase in the scale of the network necessitates a data base of a larger capacity. This leads to an increase in the scale of the server hardware.